This, pretty much. If you don't have one person with a clear understanding of the backing domain/model of the problem, you're going to be in for a lot of hurt.

This, pretty much. If you don't have one person with a clear understanding of the backing domain/model of the problem, you're going to be in for a lot of hurt. The person doesn't have to be technical, as long as they understand the business area then you just sit with them and ask them to describe things, while you translate that in your head/on paper to data models. Almost nothing else about an application matters besides your data models. The UI and other niceties are just a (very replaceable) skin around CRUD-And-CRUD-Accessories on data models. If you screw those up, you have a nice looking, useless theme. Which is pretty ironic given how much disproportionate value non-technical businesspeople place on the UI of things because they can see it/interact with it. (Not that the UI isn't massively important, but you can still interact with an API minus a frontend. The opposite scenario gives you nice-looking pictures.)